During electrophotographic processes such as xerography, it is necessary to apply a uniform level of charge to a photoconductive surface of a photoreceptor, which charge will subsequently be selectively dissipated by exposure to light, as part of the electrophotographic process. The non-discharged portions retain their charge in the form of a latent image on the photoconductive surface, and, when subsequently brought into contact with toner material, will retain toner on the photoconductive surface in the areas where the charge has not been dissipated. In a commonly used corona discharge device, (referred to hereinafter as a corotron), a high voltage in the range of .+-.5000 to 8000 volts is applied to a coronode, comprising, for example, a thin bare conductive wire or an array of pins integrally formed from a sheet metal member, supported between insulating end blocks, and mounted within a conductive channel or shield and held closely adjacent to the surface to be charged to create a corona spray which imparts electrostatic charge to the surface. In another similar device, (referred to hereinafter as a scorotron) providing more uniform charging and preventing over charging, a corona charging device is provided with a screen or control grid held at a uniform lower potential, approximating the charge level to be placed on the photoconductive surface, and disposed between the coronode and the surface to be charged.
It has been found that when using corona generating devices that produce a negative corona, desirable for use particularly with photoconductive surfaces operating with a negatively charged surface, certain difficulties may be observed. It is believed that various nitrogen oxide species are produced by the corona, and that these nitrogen oxide species are adsorbed by solid surfaces. In particular, it is believed that these species are adsorbed by the conductive shield, the housing and the screen or control grid of the corona generating device. This adsorption occurs despite the fact that, during operation of an electrophotographic machine, the corona generating device may be provided with a directed air flow to remove nitrogen oxide species as well as ozone from the area adjacent the corona generating device. After exposure to the nitrogen oxide species, when the electrophotographic machine is turned off for an extended period, and corona is not produced by the corona generating device, the adsorbed nitrogen oxide species are gradually desorbed, i.e., the adsorption is a physically reversible process. When operation of the machine is resumed, a copy quality defect is observed in the copies produced, comprising a line image deletion or lower density image formed across the photoconductive surface at the portion of the surface which was at rest opposite the corona generating device during the period of idleness.
While the mechanism of the interaction of the desorbed nitrogen oxide species and the photoconductive surface is not fully understood, it is believed that the oxide species in some way interact with the photoconductive surface, increasing lateral conductivity so that the surface cannot retain a charge in image fashion for development with toner. This causes narrow line images to blur or to wash out and remain undeveloped after toner is brought into contact with the surface. This defect, sometimes referred to as "parking deletion", increases in severity with prolonged exposure of the photoconductive surface to the desorbing nitrogen oxide species during extended periods of idleness. This problem has been observed even after a relatively short period of operation, coupled with an extended period of idleness.
During the initial stage of exposure of the photoconductive surface to the desorbing nitrogen oxide species, it is possible to rejuvenate the photoconductive surface by washing the surface with alcohol, since reaction between the surface and the nitrogen oxide species tends to remain, initially, at the surface. However, after a prolonged period of time the reaction tends to penetrate the photoconductive surface through the layers of the photoreceptor, and cannot be washed off with the solvent. The defect is reversible to some degree by a rest period. However, the period involved may be of the order of several days, which to an operator is objectionable. Frequent cleaning of the photoconductive surface is also undesirable, as it allows the possibility of damage and wear to the photoconductive surface.
Where a scorotron is used for charging a photoconductive surface, it has been found that the material from which the scorotron screen is fabricated has a significant effect on the severity of parking deletions. Heretofore, a stainless steel screen or grid has commonly been used. Other materials have been proposed, without substantial success, such as Monel, Inconel or other corrosion resistant ferrous materials which prevent the rapid oxidation of the screen material and the concurrent loss of performance characteristic of the scorotron due primarily to the corrosive effect of negative corona produced by the device. Stainless steel screens have been used primarily due to the price/performance characteristics of the material. In positive charging devices, not subject to the particularly corrosive negative corona effects, copper screening has been used to some success. To reduce the deletion problem associated with negative corona scorotron charging, considerable work has been done to reduce the adsorption of nitrogen oxide species by the stainless steel screen by applying electrodag coatings to the screen surface. Such coatings typically include a reactive metal base such as nickel, lead, copper, nickel or zinc or mixtures thereof, which tend to absorb or form harmless compounds with the nitrogen oxide species. While some success has been found using this approach, parking deletions have continued as a problem, due to the failure of the electrodag materials to continue to absorb or form harmless compounds with the nitrogen oxide species over time. Additionally, the stainless steel screen itself is somewhat expensive to fabricate, generally requiring photoetching or chemical milling to achieve the desired mechanical tolerances. These processes are, by nature, relatively expensive. High quality stamping is useful, and less expensive, requiring a first perforating and forming step, forming the screen from stainless steel sheet metal, and a second custom flattening step to achieve the high degree of flatness required for the screen. However, even with the custom flattening step, the required flatness is often not achieved. Subsequent to fabrication of the screen, the reactive metal base coatings are applied. Of course, other grid arrangements may be used, including a screen comprising closely spaced wires. However, the described arrangement offers advantages in manufacturing and operation.
Beryllium copper is known for use in pin array coronode members, such as that described for the scorotron arrangement taught in U.S. Pat. No. 4,591,713 to Gundlach et al and assigned to the same assignee as the present application. Beryllium copper has a known anti-corrosive nature, a high degree of conductivity, and is highly formable at relatively low temperatures, as described in "Beryllium Copper", Materials & Methods Manual, The Brush Beryllium Company, April, 1950. Beryllium copper is also known for good thermal stability for use as an electrode in a vacuum tube, as shown in U.S. Pat. No. 2,189,971 to Warnecke.